Since carbon fiber reinforced composite materials are light-weight and are strong with high elastic modulus, their use in sporting and leisure goods, or in space and aircraft materials are developing. Conventionally, however, since the adhesion strength between the reinforcing carbon fiber and the matrix resin is insufficient, various surface treatments to activate the fiber surface such as chemical oxidation, vapor phase oxidation, and electrolytic oxidation, have been employed. Among the, the practical is the electrolytic oxidation treatment from the viewpoint of operability and reaction controllability.
A variety of electrolyte have been studied for the electrolytic oxidation treatment. For example, Saito et al disclose in U.S. Pat. No. 4,401,533, an electrolytic oxidation process wherein a carbon fiber as an anode is treated in an aqueous sulfuric acid solution under a specified range of current, voltage, and duration.
Paul. Jr., also discloses in U.S. Pat. No. 3,832,297 an electrolytic oxidation process of a carbon fiber as an anode using an ammonium compound as an electrolyte which completely decomposes at a temperature not higher than 250.degree. C.
In U.S. Pat. No. 4,600,572, Hiramatsu et al disclose a production process of carbon fibers improved in adhesion strength with the resin wherein carbon fibers are subjected to an electrolytic oxidation in nitric acid followed by inactivation treatment.
The present inventors have suggested a two step electrolytic treatment as disclosed in Japanese Patent Application (OPI) No. 124677/86 (the term "OPI" as used herein means a "published unexamined Japanese patent application"), to effect ample surface treatment. In the previous process, however, the effect was insufficient with the carbon fibers having high modulus of not lower than 30 ton/mm.sup.2. Further, two-step surface treatments to incorporate nitrogen functional groups into carbon fiber surface are disclosed in Japanese Patent Application (OPI) No. 276075/87 by present inventors and European Patent No. 251491 (Japanese Patent Application (OPI) No. 6162/88) by S. Mitchell. A process to remove off the weak boundary layer on the carbon fiber surface on the other hand, is not given to present.
The demand for high performance carbon fiber is increasing every year. Particularly in the field of aircrafts, the development is headed for high strength and high modulus, reaching carbon fibers having middle modulus of about 30 ton/mm.sup.2 as the main current. The development trend for sports and leisure uses is similar that carbon fibers with high modulus of about 40 ton/mm.sup.2 and improved composite performance are under development. Inactivation is more used for the carbon fiber surface corresponding to the high modulus trends that the interfacial bond strength between the fiber and the resin is less effected. Conventional surface treatment process for carbon fibers is insufficient, and no optimum surface treatment process for high modulus carbon fibers to give high composite performance (particularlty ILSS, TS.perp., FS.perp., etc.) which is greatly influenced by the interfacial bond strength between the fiber and the resin, is obtained to the present.